Field of the invention
The present invention concerns binary zeolitic systems formed of two zeolites having different crystalline structures while having common structural units; it also concerns their synthesis method and their utilization.
The co-crystallization of two zeolites is a well known phenomenon in the art of zeolite synthesis. It results from the fact that in a medium favorable to their development, the zeolites are in metastable state. During nucleation or during the growth stage, several seeds can appear and develop, thus leading to the obtention of mixtures of zeolites. These mixtures generally evolve according to the rule known as the OSWALD rule which foresees that all systems evolve towards their stable state while passing through the intermediate or metastable states. By interrupting the crystallization at a given moment, this evolution ceases and it is possible to isolate a mixture of zeolite crystals in which each of the species constitutes a distinct phase easily identified by its morphology under microscopic examination. If the crystallization process is left to develop long enough, the metastable zeolites are transformed and only the stable zeolite is isolated in the medium.
The intergrowth phenomenon of the zeolite crystals is also known. It corresponds to a heterogeneous crystallization in which the crystals of a zeolite B appear sporadically during the crystallization of a zeolite A. Mircoscopic examination does not generally detect the zeolite intergrowths. These are evidenced by microdiffraction studies in which zones of the zeolite B appear as defects in the structure of the zeolite A. The best known example of intergrowth is that of the zeolite T which issues from the intergrowth of offretite and erionite. Since the intergrowth leads to a perturbation in the form and/or the size of the cages and channels, the zeolite AB obtained will have different properties from the two zeolites of which it is formed.
Both the co-crystallization and the intergrowth of the crystals appear spontaneously and do not lead to products than can be used on an industrial scale.